The long-term objectives of this project are to define at the molecular level the mechanism of action of vitamin D vis-a-vis its two principal dihydroxylated metabolites, namely 1,25-dihydroxyvitamin D and 24,25-dihydroxyvitamin D. The thesis of the proposed studies is that in terms of its chemical structure and possibly the mode of action, the secosteroid vitamin D has many similarities to that of other classical steroids. In this regard, the kidney has been identified to be the endocrine gland which produces in a carefully physiologically regulated manner small amounts of both dihydroxylated metabolites. Our biochemical efforts include characterization of the events pertaining to the regulated production of both of these dihydroxylated metabolites as well as the study of the interaction of these metabolites with their principal target tissues. Specific efforts in the current year focus on defining the biochemical properties of receptors for 1,25(OH)2D3 in the intestine as well as in a number of other heretofore unappreciated target organs, e.g. the kidney, the pancreas, the parathyroid gland, and the placenta.